It is known to provide a system which consists of a counterroll and a pressing or working roll which is urged toward the counterroll and between which a web, strip, band or discrete sheets or pieces of flexible material can be passed.
Such systems are used in the printing or coating of fabrics, in the application of flowable substances such as viscous or pasty materials or liquids to such materials and even in the pressing of such materials to smooth them or remove wrinkles therefrom.
The term "web" as used herein is intended to refer to a continuous strip or band of a flexible material, e.g. paper, fabric, nonwoven fabric or the like to be impregnated, coated, printed or smoothed, as well as to individual pieces of sheet material which can be passed in succession between the counterroll and the pressing or working roll, to strip materials and pieces of material entrained on a continuous carrier or belt between rolls and, to in general, material which can have a substantial width and which can be subjected to pressing between the rolls, i.e. materials having a relatively small thickness and a certain degree of flexibility to allow them to be impregnated or coated with the flowable substance, to allow the flexible substances to be pressed from the material or to allow the material to be pressed even in the absence of a flowable substance.
The term "flowable substance" as used herein is intended to refer to any material capable of being squeezed from or into such a web, for coating the web or impregnating the web, e.g. for printing, dyeing, conditioning, sizing or otherwise treating the web.
In machines or apparatus of the type described, i.e. having a counterroll and a pressing or working roll between which the web can pass, it is not uncommon to provide the counterroll so that it has a relatively large diameter and is axially journaled, i.e. rotatably supported at its axis or shaft, preferably at a fixed location and has relatively high bending stability (resistance to bending).
The pressing roll or working roll may have a smaller diameter than the counterroll and be urged toward the latter. It also may have a relatively smaller bending stability, i.e. a reduced bending resistance by comparison to the bending resistance of the counterroll.
In the past, with such systems, problems have been encountered in applying the pressing force via the pressing roll against the web uniformly over the entire length of the contact region and thus the width of the web. By and large, the pressing force must be relatively small if it is to be uniform across the width of the web.
The greater the diameter of the pressing or working roll and the greater the length of the rolls and the operating width, i.e. the width of the web which may also be referred to as the machine width, the greater are collateral effects and the more significant is the variability in the uniformity of the pressing force to the point that it is difficult to ascertain precisely how the pressing force will vary across the machine width and what losses may occur in the transmission of force because of undesired bending of the pressing roll.
Indeed, it has been found that roll bending is an important factor which depends upon the type of roll, its dimensioning, the materials from which it is constituted and the stresses to which the roll is subject along the length, on variation of the pressing force. Roll bending also causes problems in maintenance, requires frequent repair, increases the down time of the apparatus and contributes to problems in operation thereof.
The nonuniformity of the pressing force can give rise to nonuniformities in the product across the width thereof as well.
Accurate linearity of the rolls and a precise cylindrical configuration of the peripheral surfaces and parallelity of two rolls can only be approximated in practice. The smaller the diameter of the working or pressing roll and the greater its length, the smaller will be the pressing force which can result in bending. However, the greater the bending resistance, the greater will be the force which may be diverted from the pressing force to be lost in bending of a roll.
It is recognized that high quality results which are uniform along the full width of the web, i.e. along the machine width, will depend upon the uniform application of the pressing force over the full length of the pressing roll.
The pressing force in the past has been generated predominantly by applying a radial force at the axis of the pressing roll in the direction of the counterroll. Because of the bending tendency of the working roll, this has not been found to be fully satisfactory.